Method of manufacture of vaporelectric device



Oct. 18, 1955 G. LEWIN 2,721,108

METHOD OF MANUFACTURE OF VAPOR-ELECTRIC DEVICE Filed April 29, 1952 INVENTOR GEE/{HEP LEW/N.

' ATTORNEY United States Patent METHOD OF MANUFACTURE OF VAPOR- ELECTRIC DEVICE Gerhard Lewin, Maplewood, N. J., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application April 29, 1952, Serial No. 285,027

3 Claims. (Cl. 31621) This invention relates to vapor-electric devices such as are generally known as ignitrons, eXcitrons and the like and to the method of manufacture thereof.

In general, ignitrons are constructed with a pool cathode, usually of mercury, and therefore are not capable of universal use, but must remain in a fixed position to maintain the relationship of the pool surface constant with respect to other electrodes in the device. It therefore has not been practical to employ the usual construction of ignitron in mobile units or under conditions involving movement disturbing to the mercury pool, such as occurs with use on water, land or air vehicles. This deficiency may be met in part by employment of a sponge cathode, and in particular by placing a sponge material insert, such as of molybdenum, ruthenium, tungsten or the like, in an iron sponge, holding mercury, the molybdenum and other materials mentioned having excellent arcing characteristics without objectionable erosion. However, this is not a complete solution to the problem, since as ordinarily treated, these several mentioned materials do not readily absorb mercury and under ordinary conditions do not become wetted thereby.

Accordingly, the primary object of the present invention is to provide a metallic sponge cathode and assembly thereof in an ignitron such that it will absorb and be appropriately wetted by mercury.

A further object of the invention is to provide a method of treating a sponge cathode which will render the cathode highly eflEective and without injury to other ignitron parts in performance of the method.

Other objects of the invention will appear to those skilled in the art to which the invention appertains as the description proceeds, both by direct reference thereto and by implication from the context.

Referring to the accompanying drawing:

The figure is a central longitudinal sectional view of an ignitron of arbitrarily selected construction showing a compound sponge cathode therein as an example of utilization of my invention.

In the specific embodiment of the invention and associated ignitron construction illustrated in the drawing, but without limiting to the details thereof, a cylindrical casing of steel or other sturdy material is shown having a bottom 12 integral therewith and a top header 13 sealed at the upper rim of said casing to provide a closed envelope adapted to be evacuated.

The particular header shown, provides appropriate lead-in seals 14, 15 for electrode leads 16, 17, respectively for an anode 18 and ignitor 19 within the casing and supported from said header by said leads. The lead-in seals each include a glass or other insulating sleeve 20 by which the leads are kept electrically distinct from each other and from the casing. The anode 18 is located toward the top of the casing, well above the bottom wall 12, whereas the ignitor 19 is located below the anode, preferably centrally of the casing, and is directed toward and terminates at its bottom in the vicinity of said bottom wall. Said ignitor is shown as relatively slender and tapers downwardly, with its bottom end quite small. Said ignitor may be composed of materials as used for ignitors of the prior art, or may be of such other material or materials adapting it more especially to the present invention.

A sponge cathode, designated generally by numeral 21, is provided in said container and is of pancake shape, and situated upon the bottom wall 12 of the ignitron, preferably fitting the container at its periphery and frictionally or otherwise held permanently in fixed position. The afore-mentioned lower small end of the ignitor 19 rests upon the upper surface of the sponge cathode 21 in constant contact therewith, and as one means for maintaining such contact without detriment to the lead-in seal 15, the lead-in 17 for the ignitor, between the seal and the ignitor, is shown with a transversely extending section 22 which, with the rest of said lead-in, possesses adequate resiliency, supplemented by the weight of the ignitor, to accomplish the purpose. The ignitor projects from its contact on the sponge cathode, at right angles thereto and in a direction longitudinally of the casing. Mercury or other reconstructing liquid cathode material is applied to the sponge cathode 21 to the extent that said sponge will absorb the mercury or the like without any excess remaining on the surface. The sponge cathode 21, with the absorbed mercury, constitutes the cathode of the ignitron.

In the specific showing of the drawing, the sponge cathode 21 is constructed essentially of two difierent materials, of which one is used to take advantage of its relatively large interstices between granules for providing generous reservoir capacity, and also to take advantage of its characteristic of afiinity for and absorption of the mercury coming in contact therewith. Selection of the other essential material to constitute the sponge cathode is for purposes of providing an arestriking surface generously supplied by capillarity through said material with mercury or the like, and to take advantage of resistance to erosion of this other sponge material, during operation. More specifically, the present invention adopts a cathode 21, by way of example, having a body portion 23 essentially of sintered iron or other material with high absorptive afiinity for the reconstructing cathode fluid, such as the mercury above mentioned, and an arc-striking portion 24 essentially a sintered metal, of which molybdenum, ruthenium, tungsten and tantalum are appropriate examples.

In the specific disclosure arbitrarily selected for illustration in the drawing, the body portion 23 of the sponge cathode is shown relatively flat or thin, of pan-cake shape to fit within the bottom portion of the container fiatwise on the bottom well thereof and frictionally or otherwise held fixed in that position. The arc-striking portion 24 is shown as a smaller disc than said body portion and embedded therein concentric thereto and according to the present illustration the upper faces of the said body portion and arc-striking portion are in a common plane. The arc-striking portion is thinner than the body portion so that a part of said body portion underlies the arc-striking portion and provides a copious supply of mercury or the like to said arc-striking portion and by capillary action through the arc-striking portion the mercury is ever present at the arc-striking surface.

The body portion 23 of compressed sintered granules essentially iron, is employed in conjunction with the centrally disposed arc-striking portion 24 of compressed sintered granules of molybdenum or other material, such as ruthenium, tungsten or tantalum, to take advantage of the inherent characteristics of the iron sponge of more active absorption of mercury coming in contact therewith than would occur with a similarly compressed sintered body of molybdenum or other metals of the group having the absorptive characteristic.

One difiiculty experienced with such metals as molybdenum is that they are very difiicult to wet, especially in the presence of an ignitor, for gaseous compounds of the ignitor material and hydrogen are formed during the treatment, which react with the molybdenum or the like and render it non-wettable.

The metals used in the cathode sponge should have a low solubility in mercury, and if used in the arc region, must be refractory enough to withstand the action of the are without appreciable sputtering. High solubility as well as excessive sputtering would cause gradual sublimation of the sponge and covering of the ignitor with a conductive metal layer, thus rendering it inoperative. Unfortunately, none of the otherwise suitable metals such as iron, molybdenum, ruthenium, tungsten'or tantalum can be-easily wetted by mercury. Except for expense, iridium, platinum, and palladium may be used as sponge inserts. Iridium, platinum, and palladium are, of those mentioned, metals easiest to wet, and tantalum is the most diflicult. All of these metals are normally covered with a layer of adsorbed gas and, With the exception of noble metals, of metal oxide. Both the adsorbed gas and metal oxide are deterrents to wetting of the metal by the mercury, and are substantially eliminated in my method. Using iron as example of the sponge material, the process is carried out as follows:

First, the iron oxides are reduced by use of hydrogen.

: For this process, the hydrogen must be extremely dry to cause the amount of residual iron oxides which are in equilibrium with the water vapor, to be extremely small. Second, any adsorbed oxygen must be replaced by hydrogen; and thirdly, the adsorbed hydrogen must be replaced by mercury. I have found that, if the sponge is cooled in hydrogen, the gas pumped out, and mercury admitted, the mercury readily wets the ironwith a simultaneous rise in gas pressure which is assumed as caused by the release of the adsorbed hydrogen. Evidently, the affinity of the mercury to the iron is large enough to replace the hydrogen.

If molybdenum is treated this way, it will be wetted only partially or not at all. An additional heating in vacuum is required to remove the adsorbed hydrogen. The treatment performed by me to obtain wetting of the molybdenum sponge consists of a vacuum outgassing for approximately one hour at a temperature higher than any subsequently reached (800900 C.), followed by a hydrogen treatment as above described and subsequent vacuum out-gassing for one hour each at about 750 C.

In case of a combination sponge consisting of molybdenum and iron, the iron will be wetted immediately after heating and cooling in hydrogen regardless of the hydrogen pressure during mercury admission, while the molybdenum can only be wetted after an additional vacuum heating.

.With a combination sponge of iron and ruthenium, surprisingly neither the iron nor the ruthenium can be wetted after the hydrogen treatment but a thorough su sequent vacuum bake as used for molybdenum is required to render both wettable. Apparently, the presence Higher temperatures and/or longer times are required.

The surface of an iron or molybdenum sponge filled at the surface, thereby rendering further wetting impossible. The mercury originally contained on the oxygenated part of the surface recedes to the interior and some mercury oozes out elsewhere through the yet undamaged portion of the surface. If an originally wetted iron sponge is partially depleted of its mercury under vacuum and subsequently exposed to the atmosphere, small droplets of mercury can be seen leaving the sponge at high velocity but at relatively slow rate. lnthis case, the attack obviously takes place at the internal surfaces of the sponge as well and may cause a rather sudden reversal of capillary forces there; the exact mechanism of the'process, however, is not very well understood. It is therefore necessary to apply my process directly within the ignitron.

The ignitron is first vacuum-baked in a two part furnace, the portion around the anode being held at 910 C. and the lower part including the cathode header, at 815 C. The anode and ignitor seals are kept cool by air'blasts. This bake-out period lasts three hours Whereupon temperatures are reduced to 770 C. The remainder of the exhaust process serves to render the sponge wettable: First the tube is filled with an oxygenfree, very dry (dew-point below-100 C.) gas mixture containing 80% helium and 20% hydrogen. (Helium is added to render the gas non-explosive.) The oxygen content of the gas is converted into water vapor by a palladium catalyst and the water is removed by a Lectro- Dryer and finally by cooling with liquid nitrogen. Purification stages separate the main part of the system from rubber hoses to avoid contaminations originating from the rubber. To intensify flushing of the internal surfaces of the sponge, the gas is continuously renewed for one hour by pumping out and refilling by means of an automatically operated valve. The cycle is: roughly exponential pressure reduction to mm; taking 20 seconds, followed by a linear pressure rise to 700 mm. taking seconds. After the gas treatment, the tube is subjected to a 10- mm. of mercury vacuum'bake at 770 C. for

one hour to drive out the adsorbed and probably some absorbed hydrogen. Helium being a noble gas is hardly adsorbed at or above room temperature. Then, the upper part of the furnace is turned off allowing the anode portion of the tube to cool, while the sponge portion of the tube is maintained at 770 C. After another hour, when the anode temperature has dropped below 600 C., the bottom portion of the'furnace is shut off and allowed to cool slowly so that the sponge portion of the tube will be, at all times, at least as hot as any 7 other tube part. This differential cooling is necessary to avoid readsorption by the sponge of gas released from other tube parts, particularly the graphite anode. The recommended values, however, of times and temperatures involved may be modified in order to obtain optimum results. At about 300. C. as soon as the rate of gas desorption from the tube is small enough to per mit, while pumping, pressure reading on the ionization gauge of less than 5 x 10 mm., cooling is accelerated by filling the tube with pure neon to a pressure of 30 mm. When the tube temperature has dropped to about 200 C., thesponge is filled and flooded with mercury which has been previously vacuum distilled and which is introduced through the exhaust tubulation. The cathode header of the tube is then heated with a gas burner until the mercury boils for a few'minutes, whereupon the excess mercury is removed. The boiling provides a more intimate contact between the mercury and sponge which facilitates thorough wetting and should remove traces of adsorbed or absorbed gas (mainly hydrogen) from the mercury. To avoid distillation of some of the mercury from the filled sponge, the neon is pumped out only after cooling the cathode header below 60 C. The tube is then sealed off.

Finally, I wish to point out that the hydrogen adsorbed on the sponge surface is probably bound weakly to the iron sponge in the form of molecular hydrogen by socalled Van der Waal forces, and that with the other metals, such as molybdenum, ruthenium, tungsten and tantalum, this bond is much stronger. The fact that even the iron becomes non-wettable if hot ruthenium or tungsten is present during the hydrogen treatment, indicates that the atomic hydrogen which is formed at the surface of these metals, is bound as such at the iron surface as well.

I claim:

1. The method of manufacture of ignitrons having an assembly of an envelope containing an anode at one part thereof and a sponge cathode at another part thereof, comprising filling the envelope with dry hydrogen and thereafter evacuating the same, vacuum baking the assembly, cooling the envelope at the anode-containing part thereof while maintaining a higher temperature of the cathode-containing part of said envelope, filling the envelope with an inert gas, flooding the cathode with mercury, boiling the mercury, evacuating the inert gas, and sealing the envelope.

2. The method of manufacture of ignitrons having an assembly of an envelope containing an anode at one part thereof and a sponge cathode at another part thereof, comprising heating the anode and cathode, filling the envelope with dry hydrogen and thereafter evacuating the same, vacuum baking the assembly, cooling the envelope at the anode-containing part thereof while maintaining the temperature of the cathode-containing part of said envelope, filling the envelope with argon, flooding the cathode with mercury, boiling the mercury, evacuating the argon, and sealing the envelope.

3. The method of manufacture of ignitrons having an assembly of an envelope containing an anode at one part thereof and a sponge cathode at another part thereof comprising differentially heating the anode and cathode and envelope therefor, repeatedly filling the envelope with dry hydrogen and evacuating the same, vacuum baking the assembly, cooling the envelope at the anodecontaining part thereof While maintaining the temperature of the cathode-containing part of said envelope, filling the envelope With argon, flooding the cathode with mercury, evacuating the argon, and sealing the envelope.

References Cited in the file of this patent UNITED STATES PATENTS 2,547,536 Pollard Apr. 3, 1951 2,617,065 Lewin Nov. 4, 1952 2,627,481 Lewin Feb. 2, 1953 

1. THE METHOD OF MANUFACTURE OF IGNITRONS HAVING AN ASSEMBLY OF AN ENVELOPE CONTAINING AN ANODE AT ONE PART THEREOF AND A SPONGE CATHODE AT ANOTHER PART THEREOF, COMPRISING FILLING THE ENVELOPE WITH DRY HYDROGEN AND THEREAFTER EVACUATING THE SAME, VACUUM BAKING THE ASSEMBLY, COOLING THE ENVELOPE AT THE ANODE-CONTAINING PART THEREOF WHILE MAINTAINING A HIGHER TEMPERATURE OF THE CATHODE-CONTAINING PART OF SAID ENVELOPE, FILLING THE ENVELOPE WITH AN INERT GAS, FLOODING THE CATHODE WITH MERCURY, BOILING THE MERCURY, EVACUATING THE INERT GAS, AND SEALING THE ENVELOPE. 